1. Field of the Invention
The present invention relates to a variable displacement pump employed for a power steering device or the like of a motor vehicle.
2. Description of the Related Art
Conventionally, there has been a variable displacement pump as disclosed in Japanese Patent No. 2932236 in order to assist steering force by means of a hydraulic power steering device of a motor vehicle. This conventional variable displacement pump is directly rotated and driven by means of an engine of the motor vehicle. This device provides a rotor in a cam ring engaged movably and displaceably with an adapter ring engaged with a pump casing, and forms a pump chamber between the cam ring and the periphery of the rotor.
Further, in this conventional art, as well as the cam ring is structured such as to be movable within the adapter ring and a biasing force making a capacity of the pump chamber maximum is applied to the cam ring by a spring, first and second fluid pressure chambers are separately formed between the cam ring and the adapter ring and there is provided a switch valve operating due to a pressure difference between upstream and downstream sides of a main throttle provided in a pump discharge side passage and controlling a fluid pressure supplied to both of the fluid pressure chambers in correspondence to a discharge flow amount of a pressurized fluid from the pump chamber so as to move the cam ring, thereby changing the capacity of the pump chamber so as to control the discharge flow amount from the pump chamber. Accordingly, in this variable displacement pump, the discharge flow amount is controlled to be relatively large so as to obtain a large steering assist force at a time when the motor vehicle stops or runs at a low speed, where the motor vehicle has a low rotational speed, and the discharge flow amount is controlled to be equal to or less than a fixed amount so as to make the steering assist force small at a time when the motor vehicle runs at a high speed, where the motor vehicle has a high rotational speed, whereby it is possible to generate the steering assist force required for the power steering device.
In this case, in the conventional art (Japanese Patent No. 2932236), since an opening range around a pump shaft of a discharge port open to a discharge area in a downstream side in a rotor rotating direction of the pump chamber is arranged so as to be shifted to a side of a second fluid pressure chamber, a force based on a pressure fluctuation (an increase of internal pressure of a cam ring) generated within the pump chamber moves the cam ring to a side of the second fluid pressure chamber so as to fluctuate the discharge flow amount of the pump at a time when a load is generated on the basis of operation of equipment to be used, such as a steering operation of a power steering device or the like. In Japanese Patent No. 2932236, it is described that since the fluid pressure in the downstream of the main throttle is substantially close to the discharge pressure which can resist against the increase of the internal pressure of the cam ring mentioned above, when this pressure is introduced into the second fluid pressure chamber, the movement mentioned above of the cam ring can be restricted by the introduction pressure, and the fluctuation of the flow amount mentioned above can be prevented. However, this description is an error. It is impossible to prevent the flow amount from being fluctuated.
Because the force (except the spring) applied to the cam ring is constituted by the fluid pressure of the first fluid pressure chamber, the second fluid pressure chamber, and the pump chamber, the fluctuation of the pressure is transmitted to all the area of the discharge system from the pump chamber to the equipment in use when the load is generated. At this time, since the force based on the pressure fluctuation generated in the first fluid pressure chamber and the force based on the pressure fluctuation generated in the second fluid pressure chamber have substantially the same area in their pressure receiving surfaces and are opposed to each other, they are cancelled with each other. However, the force based on the pressure fluctuation generated in the pump chamber leaves as before. This force moves the cam ring to the side of the second fluid pressure chamber so as to fluctuate the flow amount.
The object of the present invention is to restrict a fluctuation of a discharge flow amount when a load is generated, in a variable displacement pump.
According to the present invention, there is disclosed a variable displacement pump comprising: a rotor rotated and driven in a state of being fixed to a pump shaft inserted to a pump casing and receiving a multiplicity of vanes in a groove so as to be movable in a radial direction; a cam ring fitted to a fitting hole in the pump casing so as to form a pump chamber between the cam ring and an outer peripheral portion of the rotor, making it movable within the pump casing and forming first and second fluid pressure chambers between the cam ring and the pump casing.
An opening range around a pump shaft of a discharge port open to a discharge area in a downstream side in a rotor rotational direction of the pump chamber is shifted to a side of the second fluid pressure chamber.
A variable throttle passage and a fixed throttle passage bypassing the variable throttle passage are provided in the discharge passage of the pump.
A pressurizing cylinder is provided in an opposite side of the first fluid pressure chamber, holding the cam ring there between, and a piston inserted to the pressurizing cylinder collides with the cam ring.
Pressure in an upstream side of both throttle passages comprising the variable throttle passage and the fixed throttle passage provided in the pump discharge side passage is introduced to the first fluid pressure chamber and the oil chamber of the pressurizing cylinder. Pressure in a downstream side of the both throttle passages is introduced to the second fluid pressure chamber.